This invention relates to capacitive liquid level sensors and, more particularly, to capacitive sensors capable of sensing the interface between different layers, including air. Such liquid level sensors find use in many instruments wherein a robotic probe is used to withdraw liquid from a container containing a sample to be analyzed or a reagent.
In such robotic systems, it is desirable to have knowledge of the level of the liquid or the interface between liquids in the container such that the probe used to withdraw the liquid can be controlled (1) to withdraw a particular layer of liquid in the container or (2) to minimize contact with an undesired portion or layer of the liquids in the container. In such systems one is dealing with generally immiscible liquids such as occurs in the collection of blood. In a typical blood system producing packed red blood cells the red blood cells will be in the lower portion of the container. Immediately above the packed cells is a commercial separation gel. Above the separation gel is the serum or plasma and finally air on the top. Contamination of the sampling probe by the separation gels is very undesirable. Often it is relatively difficult to remove the gel from the probe and can in fact cause clogging of the probe and missampling to occur. Therefore it is highly desirable to provide some system of locating the gel-serum interface so that the serum only can be withdrawn and the probe prevented from contacting the gel.
To accomplish this objective, it is necessary to be able to sense the level of the liquid interfaces at all times on a real time basis. Various level sensors have been developed for this purpose. Among those are the so-called capacitive level sensors. These are based on the fact that any conductor exhibits a finite electrical capacitance. This capacitance, when approaching a liquid having a higher dielectric constant, will increase. When the sensing probe is in close proximity to a liquid, the higher dielectric constant and greater surface area results in an increased capacitance of the probe. These capacitance changes caused by the liquid can be rather small so that sensitive detection devices are required.
Devices known in the prior art that are suitable for detecting small changes in capacitance include bridges, RC or LC oscillators and frequency meter counters (including heterodyning), phase locked loops, zero crossing periodometers, amplitude changes to an RC or LC filter, and phase shift changes through an RC or LC circuit.
Among the prior art capacitive liquid level sensors are Kingston U.S. Pat. No. 3,391,547 which discloses a capacitive liquid level probe for a liquid tank. He utilizes a capacitor probe, disposed in the liquid, as one leg of a bridge circuit. An unbalance in the circuit, as a result of change in capacitance of the probe, is detected by a phase sensitive detector which is referenced by the fixed frequency excitation oscillator through a variable phase shifter. The variable phase shifter allows for offset adjustment.
In similar manner, Oberli U.S. Pat. No. 3,635,094, discloses a capacitive level sense means for an automatic transfer pipette. The sample probe is utilized as the first element and a metal stand around the sample vessel is the second element which forms a capacitor in one leg of a bridge circuit. The remaining legs of the bridge consist of a variable capacitor leg and two resistor legs. The variable capacitor leg may be adjusted such that its capacitance matches that of the probe contacting the liquid. The bridge circuit is excited by a fixed frequency oscillator and a differential amplifier is utilized to determine when the bridge is balanced indicating that the probe has contacted the liquid.
Bello et al. U.S. Pat. No. 4,326,851 discloses a level sense apparatus and method for use in an automatic clinical analyzer in which a variable capacitor is formed by a grounded probe and a metal plate, which is connected to the detection circuit, disposed below the sample vessel. A fixed frequency excitation signal is utilized and the capacitance change resulting from the probe contacting the liquid is detected as a voltage change in the detection circuit. This arrangement presents a problem in that spills on the electrode or supply tray can change the circuit operation and the circuit requires the use of shielding pads.
Another U.S. patent, Okawa et al. U.S. Pat. No. 4,736,638 discloses a liquid level sense apparatus for use in an automatic clinical analyzer. A metal plate disposed under the sample vessel and connected to a fixed frequency oscillator emits low frequency electromagnetic radiation up through the sample. The dispense probe serves as an antenna and is connected to a detection circuit, having appropriate bandpass filtering, which detects a voltage amplitude change when the probe contacts the liquid sample. This circuit has many of the disadvantages of Bello. In addition, the use of low frequency limits the time response of the circuit.
Finally, Shimizu U.S. Pat. No. 4,818,492 discloses a capacitive liquid level sensor for an automatic clinical analyzer. He utilizes a resistor bridge with a fixed frequency oscillator exciting one diagonal of the bridge and the probe serving as a capacitor across the other diagonal. Phase shift across the capacitor (probe), as a result of change in capacitance of the probe, is detected by a phase detector which is referenced by the fixed frequency excitation oscillator through a variable phase shifter. The variable phase shifter allows for offset adjustment. The output of the phase detector is filtered and compared against a reference value to provide a signal indicating the presence of liquid at the probe.
None of these sensors are directed to sensing the liquid interfaces in any useful fashion as the probe must disturb such interfaces as it journeys down through the tube or container. To solve this problem, various systems have been devised which seek to determine liquid level from the exterior, of a container. Typical of these systems are those described in U.S. Pat. No. 4,099,167 and U.S. Pat. No. 4,002,996. In both of these systems electrodes are disposed on the exterior of the container and changes in the dielectric provided by the contained liquid as compared to air is sensed by causing a variation in a capacitance sensitive detector. Another system such as that described in U.S. Pat. No. 4,371,790 uses the electrical conductance of a liquid to determine the level of the liquid contained in a container.
Finally, U.S. Pat. No. 3,939,360 describes a similar system in which a tape is attached or fixed to the outside of a container whose liquid level is to be sensed. Unfortunately such system, as are the others of the prior art, is relatively inaccurate in sensing the location of the liquid interfaces and are unable to seek the level of liquid/air interface but must allow the interface to pass by its location before such is detected.
Optical sensors can be used, but they often are impractical for the simple reason that paper or other labels are usually affixed to the outside of the container and hence would prevent optical scanning. Furthermore, if the outside of the container is dirty as for example with dried blood, the optical ability to sense could be considerably decreased. Also any water that may condense on the exterior surface would interfere with optical sensing. This would be a particular problem when the sample has been refrigerated.